Chlorobenzenes dechlorination

The cyanobacteria Anflfcflenfl sp. strain PCC 7120 andNostoc ellipsosporum dechlorinated y-hexachloro[flflfleee]cyclohexane in the light in presence of nitrate to y-pentachlorocyclo-hexene (Figure 2.5), and to a mixture of chlorobenzenes (Kuritz and Wolk 1995). The reaction is dependent on the functioning of the nir operon involved in nitrite reduction (Kuritz et al. 1997). 

Compare the result with the values given for dechlorination of other chlorobenzenes in Table 14.3. Comments ... 

Silica- or alumina-supported Pd-Fe mono- or bimetallic dehalogenation catalysts were prepared by conventional and microwave-assisted calcination. The catalysts synthesised under microwave conditions gave a higher conversion in the dechlorination of chlorobenzene. The effects were attributed to an enhanced crystallite size, a lower susceptibility to alloy formation and differences in the Pd morphology73,74. 

The catalytic hydro dechlorination of chlorobenzene has also been studied on Pd/Nb205 catalysts. Also in this case, the catalysts prepared under microwave irradiation are more resistant towards deactivation than the corresponding catalysts prepared under thermal conditions75. 

To analyze the reactivity of organics in systems using zero-valent iron, AH, (standard heat of formation) and ELUMO (energy of the lowest unoccupied molecular orbital) were computed using the PM3 Hamiltonian method. Reductive dechlorination rate constants for five chlorobenzenes in the pres-... 

The intramolecular photoinduced electron transfer reaction of N-(o-chlorobenzyl)aniline 440 in the presence of sodium hydroxide in aqueous acetonitrile afforded, 9,10-dihydrophenanthridine and its dimer, which is reasonably explained by dechlorination from the radical anion of chlorobenzene chro-mophore followed by the cyclization (Scheme 130) [481], Similar photocyclization 9-(io-anilinoalkyl)-10-bromophenantherens 441 takes place to give spiro compounds, cyclized products, and reduction products dependent on the methylene chain length. The efficient intramolecular photocyclization occurs when the methylene tether is n = 3 [476] (Scheme 131). 

In order to determine the reaction pathway for 1,2-dichlorobenzene, Schiith fitted kinetic data and found that the primary pathway was direct reaction to benzene with a parallel reaction of sequential dechlorination through chlorobenzene to benzene. (Figure 7) These pathways were further supported by independent determination and verification of the reaction rate constant for the second hydrodechlorination step. (Schiith and Reinhard 1998)... 

Chlorinated hydrocarbons have also been reduced on Cu cathodes in aqueous solutions [7]. In this case, a fixed-bed, flow-through reactor filled with Cu balls, 0.2-0.6 mm in diameter, supported on a Pt gauze was employed. Hexachlorocyclohexane was dechlorinated rapidly and completely. Tetrachloroethylene, trichloroethane, and chlorobenzene were less reactive. However, unsatisfactory results were obtained with a polychlorinated biphenyl (PCB). 

The radical anion can participate in electron transfer reactions, in which the electron is transferred to a compound with suitable electron affinity. The parent aromatic compound is regenerated. Such a reaction is shown in Eq. (83), where chlorobenzene is dechlorinated by electron transfer from the benzene radical anion ... 

Chlorinated aromatics such as chlorobenzene may also be dechlorinated by solvated electron capture as shown in Eq. (84). Aromatic halogenated organics are readily reductively degraded by electron capture. Typically, sequential dechlorination occurs until the aromatic skeleton remains in unhalogenated form. The chlorine-containing product is HC1. 

Acetone-sensitized and unsensitized reductive dechlorination of tetra-, penta- and hexa-chlorobenzenes have been studied373,374, 1,2,4-Trichlorobenzene 125 is the major photoproduct from 1,2,3,5-tetrachlorobenzene 122 in the unsensitized process, whereas the... 

Chlorobenzene, 1,2,3,5-tetrachlorobenzene and lindane (y-hexachlorocyclo-hexane) were dechlorinated in different solvents on plain and metal-modified carbon cloth cathodes. In dimethylsufoxide and ACN, chlorobenzene was dechlorinated even on plain carbon materials, while Pd-coated and Zn-coated cathodes were necessary to perform degradation in methanol and water, respectively (Kulikov etal. 1996). 

The use of metal-complex mediators was applied by Nunnecke and co-authors (Nunnecke and Voss 1999) to the dechlorination of chlorinated benzenes and diben-zofurans. The main products were chlorobenzene and unsubstituted dibenzofurans, respectively. 

Bromobenzene is hydrogenolyzed in a much greater rate than chlorobenzene over Pd-C in methanol. The rates are further increased by added potassium acetate for both bromo- and chlorobenzenes.196 In one patent, sodium phosphate was used as an effective base in the dechlorination of 4,6-dichloro-2-nitroresorcinol to 2-aminoresorcinol over Pd-C.234... 

Lahaniatis et al. found that a thermal degradation of toxaphene at 400°C to 800°C leads to the formation of aromatic compounds, among them tri- to hexa-chlorobenzenes, low-chlorinated naphthalenes, biphenyls, and even dibenzofu-rans [176]. The major processes of degradation seem to be dechlorination and dehydrochlorination [54]. However, Chandurkar and Matsumura reported that at least some hydroxylated toxaphene metabolites are formed [81]. So far, only very few oxidation products of toxaphene have been identified [177,178,179] which seem to be rather unstable [177]. 

An interesting result has been obtained in a study of the dechlorination of polychlorobenzenes by direct irradiation in acetonitrile in addition to dechlorination, products resulting from rearrangement of the chlorine atoms to positions mefa to their original point of attachment are observed. Thus orfho-dichlorobenzene was found to yield mainly chlorobenzene along with small amounts of para-dichlorobenzene the authors speculate that the rearrangement products are formed by recombination of an aryl-chlorine radical pair. In another novel result photolysis of pentachlorophenol in acetonitrile is found to give small amounts of the benzoxazole (160). ... 

A palladium-modified carbon-cloth cathode in MeOH containing TEABr has been used by Kulikov and coworkers [176] for the stepwise dechlorination of 1,2,3,5-tetra-chlorobenzene and by Tsyganok and Otsuka [202] to carry out the dechlorination of 2,4-dichlorophenoxyacetic acid (a herbicide). 

There are only few examples that are initiated or facilitated by the excited singlet of anilines. As mentioned earlier, this could be attributed to its weak absorption at long wavelength that limits the choice of substrates as ground-state acceptors. Previously, photosensitized dechlorination of chlorobenzenes by A,A-dimethylaniline (lb) was found to be efficient ( = 0.2 - 0.6)242,243. In accord with the fluorescence quenching behavior of anilines by chlorobenzenes244, the dechlorination process occurs by electron transfer from the excited lb to the chlorobenzene, followed by fast rupture of the C—Cl bond. 

The photolytic removal of chlorine from chlorobenzene, chlorophenols and chlorinated biphenyls and dioxins continues to be examined as a potential solution to the problem of the destruction of these compounds when present as environmental pollutants. Guillet has reported that aqueous solutions of the copolymer of vinyl-naphthalene and styrene sulphonate will solubilize to a small degree 2,2, 3,3, 6,6 -hexachlorobiphenyl, and that illumination of the solution with simulated sunlight leads to the formation of biphenyls with fewer chlorine substituents. It is suggested that the process involves the absorption of light by the naphthalene and exciplex formation with the biphenyl followed by electron transfer to the biphenyl. The chlorinated biphenyl anion radical would then be expected to expel chloride ion. The dechlorination of mixtures of variously chlorinated biphenyls (such as those typically used as... 

Chemical decontamination is an alternative to thermal processing or landfilling of soils contaminated with polychlorinated dibenzo-p- dioxins (PCDD) or other aromatic halides such as chlorobenzenes or polychorinated biphenyls (PCB). Chemical decontamination, like incineration, involves changes to the chemical structure of the dioxin molecule. While chlorinated dioxins are thermally stable, they readily dechlorinate to water soluble compounds under relatively mild conditions of temperature and pressure. 

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